Capacitive sensing is a technology based on capacitive coupling, a capacitive sensor can detect characteristics of materials nearby a capacitive sensor. The materials may either be conductive and/or have a different dielectric than its surroundings. Capacitive sensing is used in many different types of sensors, including those to detect and measure proximity, position or displacement, humidity, fluid level, and acceleration. Capacitive sensing has become more popular as capacitive sensors for detecting changes in capacitance become more accurate and reliable. For instance, capacitive sensors are used in many devices such as laptop trackpads, digital audio players, computer displays, mobile phones, mobile devices, tablets, etc. Design engineers continue to choose capacitive sensors for their versatility, reliability and robustness, and cost reduction over mechanical switches.
Capacitive sensors typically work by providing an electrode on a single- or multiple-layer printed circuit board (PCB)), flex circuit, and/or printed conductor, where the electrode is stimulated to generate an electric field. The sensor electrode on the PCB forms as one plate of a virtual capacitor. The other plate of the virtual capacitor is provided by an external object, such as a user's finger, or some kind of conductive material, etc. The sensor electrode can measure changes to the capacitance over the virtual capacitor over the sensor electrode as the external object disturbs the electric field generated by the sensor electrode. Using the capacitive measurements, disturbances caused by the external object can be accurately detected.
Overview
The present disclosure describes a differential shield capacitive sensor design. The sensor design uses a differential measurement to measure capacitance and a pair of traces are used to differentially reject the response of the sensor traces and balance any parasitic capacitances. In some embodiments, the sensor design includes a differential sensor design on a bottom side of a flex circuit to differentially balance the environment and reject noise coupling to the sensor. The top side of the flex circuit can include a single ended design for proper environment sensing. The spatial arrangement and size of the sensors may vary depending on the application.